Police say Edgar Maddison Welch traveled from North Carolina to Washington and searched the pizza restaurant with an assault rifle, terrifying employees and customers and shooting the lock off a door before concluding that there were no sex slaves hidden within.

The “Pizzagate” conspiracy theory, as it has come to be known, appears to have grown out of message board chatter surrounding the WikiLeaks release of emails from Clinton campaign chairman John Podesta’s account. Those emails included conversations between Podesta and Comet owner James Alefantis.

Discussions that began on 4chan and quickly spread to Reddit and social media spun those emails into a baseless conspiracy about a child sex trafficking ring involving prominent Democrats. Soon, tens of thousands of people were discussing the supposed involvement of Comet Ping Pong in this imaginary sex trafficking ring and the restaurant was receiving threats.

Friends and family members have told reporters they are unsure how Welch became fixated on the story, but many observers who have been trying to sound the alarm about fake news for months said his alleged actions show how dangerous the spread of misinformation can be.

“I don’t think there is an agreed-upon definition of what’s fake news and what’s not, and part of the reason is because many people seem to be living in alternate realities at this point,” said John Carroll, a former journalist and professor of mass communication at Boston University.

When people are convinced of claims that have no basis in fact like Pizzagate, disproving them becomes an impossible task.

“The people who believe that there was human trafficking out of a pizza shop in Washington, D.C., you can tell them that’s not happening, but they continue to believe that it is,” Carroll said. “It turns out the truth isn’t necessarily the antidote in some of these cases.”

A new Morning Consult poll illustrates how prevalent concerns about the issue have become. Overall, 70 percent of respondents had heard about the “fake news controversy,” while nearly half said they were exposed to fake news on social media at least once a day.

About 70 percent of respondents said it would be appropriate for Google, Facebook and Twitter to remove fake news stories, and 67 percent said web service providers should remove them. Still, only 17 percent said social media platforms are most responsible for ensuring people are not exposed. Nearly a quarter said the person reading the news is most responsible.

Although many polls have found the media in general faces severe credibility problems with the American people, the survey found 60 percent say CNN is a credible news source and 67 percent say ABC is. The Wall Street Journal is seen as credible by 64 percent, while 63 percent said frequent Trump target the New York Times is credible. Only 19 percent said the pro-Trump Breitbart News is a credible source.

In the current partisan environment, “fake news” has quickly become a buzzword for any story that a reader deems to be incorrect or does not call out information they disagree with explicitly enough.

Broadly, the stories that have come under the growing umbrella of “fake news” in recent months falls into several categories:

Conspiracy theories

Unlike willfully false reporting posted with the intent to mislead, some nonsensical stories are written by people who genuinely believe them and want to convince others. Many promoters of Pizzagate seem deeply concerned that this fictitious sex trafficking is going uninvestigated.

Message boards

The crowdsourcing of information has its advantages, but it carries risks as well. The Pizzagate story has been traced back to message boards where suspicions about a few emails spiraled quickly into an utterly ludicrous story. In the past, message boards have led to the misidentification of one of the Boston Marathon bombing suspects and other errors.

Satire

Apparently New Yorker humor columnist Andy Borowitz’s work is at times mistaken for fact. A recent FactCheck.org post noted that the site has received many inquiries from people who read satirical columns, thought they were far-fetched, but just wanted to check to be sure. Sarcasm and satire do not always translate well across social media, so such misunderstandings may be unavoidable.

Partisan news sources

Disdain for hyper-partisan news outlets that twist facts nearly to their breaking point is often well-deserved, but it is a different phenomenon. A recent Buzzfeed news analysis of Trump’s tweets found that the most popular stories he has shared came from explicitly conservative news sites like Breitbart, the Daily Caller, the National Review, and Lifezette. Some make dubious claims, but none could be called outright unsubstantiated lies.

Inaccurate reporting

In the blowback against perceived fake news hysteria, some have taken to declaring any legitimate reporting that turns out to be incorrect “fake news.” Mistakes happen in journalism, and while they do nothing to help the media’s credibility, those errors are a far cry from imagining stories out of whole cloth.

Accurate reporting you don’t like

Not every Fox News story about the Clinton Foundation is false and not every New York Times report delving into Donald Trump’s ethical and legal conflicts is a lie. Attempting to dismiss legitimate investigation as “fake news” makes it harder to discredit the real fake stories when they surface.

As the Pizzagate example demonstrates, a fake story can begin its life in one of these categories and work its way through others as it is reposted, reported, and shared across the web.

The focus on Trump supporters circulating these stories and a perception that “fake news” is becoming a scapegoat for Hillary Clinton’s failings as a candidate has led to a pushback from some conservatives.

“It’s a way to marginalize all nonliberal voices and blur the lines between viral sites pushing questionable content and reliable outlets with which we may just disagree,” Karol Markowicz wrote in the New York Post.

Former Fox host Greta Van Susteren argued in a Los Angeles Times column that “one of the bigger fake news stories of this decade” was the Obama administration’s claim that the attack on a U.S. diplomatic compound in Benghazi was the result of a protest over an anti-Muslim YouTube video. That claim was based on what later proved to be inaccurate intelligence assessments.

Although some producers of fake news have said they found a particularly fertile audience in Trump supporters and their attempts to bait liberals with similar tactics failed, others say there are plenty of misconceptions and conspiracy theories held by the left as well.

Some experts worry that fake news will grow even harder to refute in the coming years with Trump bashing the mainstream media at every opportunity and frequently spreading false stories on Twitter and in rallies himself.

“When the source of some of this fake news is the White House and the Trump administration, that’s going to give the fake stories more legitimacy in the eyes of many people,” Carroll said. “The very fact that the mainstream media will be questioning those stories only reinforces the attachment to them by the people who believe them.”

Trump’s baseless claim that three million people voted illegally in the election appears to be born of Infowars conspiracy theories. His aides and allies, including Vice President-elect Mike Pence, appeared on Sunday talk shows this week defending his unsubstantiated voter fraud allegation.

Designated National Security Advisor Michael Flynn and his son have come under fire since Sunday’s incident for their propagation of lies about Clinton-linked sex trafficking. A Politico report Monday found that Lt. Gen. Flynn had tweeted questionable or outright false information 16 times since August, including claims that President Obama is a jihadi money launderer and Podesta consumed bodily fluids in occult rituals.

According to misinformation researcher Filippo Menczer of Indiana University, the creeping advance of fake news across social media is a symptom of users retreating into their own "information bubbles," seeking news that confirms their existing views and blocking out contradictory information. As long as that impulse is present and information that caters to that desire is so readily available, the problem is unlikely to go away.

This is where the shifting definition of “fake news” again becomes problematic, though. How false does a story need to be to be considered fake? Is it based on the content or the source? How do you avoid allegations of partisanship if you start censoring content that targets one political figure or another?

“There may be a solution, but I’m not smart enough to know what it is,” Carroll said. “This seems to be a self-perpetuating cycle that only gets worse as people narrow their information universe.”